I would like to connect the Atmega328p chip to my desktop, through the USB, so I can run teraterm or hyperterminal for debugging purposes. I have already ordered a USB to TTL serial cable but it won't be arriving for about a week. I recently bought a usbASP ISP to program the chip on my Arduino. My question is: can I use the usbASP in place of the TTL cable? or is there some conditioning circuit in the usbASP that won't allow this?

It seems for the usb to ttl, all that is required to interface with the chip is VCC, GRD, RX and TX. So, which pins are the RX and TX?

Hope my question makes sense, they usually don't! thanks in advance!

  • For simple debugging you could just add a few leds. Optionally make them blink in certain patterns for more complex information.
    – Gerben
    Commented Feb 28, 2016 at 18:25

1 Answer 1


There is provision in the USBasp specification for a UART connection, however, not all manufacturers copy the original schematic exactly, and no firmware actually contains the appropriate code for CDC/ACM required in order to enable it. And even if they did, technically low-speed CDC/ACM is a violation of the USB protocol and not all operating systems will acknowledge it.

As for your title of "Debugging with ISP", if you have an Atmel programmer and are using Atmel Studio then you can perform debugging over ISP (actually debugWIRE, but they use the same connections).

  • So i can output messages to Atmel Studio? Say, outputting a voltage as a turn a pot, from the adc? Commented Feb 28, 2016 at 18:07
  • 1
    No. But you can use debugWIRE to inspect the register contents when the code hits a breakpoint. Commented Feb 28, 2016 at 18:11

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